The present invention relates to an extrusion cover for containers.
A container for which a cover of this kind is used is interchangeably set in an extrusion gun 1 as shown in FIG. 1.
When a lever 2 of the gun 1 is intermittently pressed, a rod 3 is forwardly driven to allow a piston 4, which is attached to the front end of the rod 3, to break through a rear cover 6 provided on a container 5. As a result, a central portion 7 of the rear cover 6, which is separated from a circumferential portion thereof by the piston 4, and which is in contact with the front end surface of the piston 4, causes an extrusion piston 8 fitted in the container 5 to be driven in the forward direction as shown in FIG. 2. The material 9 in the container 5 is thus squeezed out from an outlet 10 at the front end of the container 5 by the pressure applied to the material 9 by the extrusion piston 8.
The material subjected to extrusion in the abovedescribed manner is a dampproof, permanently elastic sealing material, such as polyurethane, which is used to fill therewith gaps occurring in buildings, electric cars, ships, airplanes, internal combustion engines, chemical machines, etc. for the purpose of preventing the leaking of rain and mixing of solvents into the structural bodies.
Since such a material is in the form of a semifluid in the container and since the material is hardened when it is brought into contact with the atmospheric air, it is necessary that the container be in a perfectly sealed state even when it is in a natural state before use.
A polyurethane sealing material referred to above is so easily affected by the atmospheric air that it cannot be assured more than several months with respect to its characteristics even if it is stored according to a regular method before use. Therefore, it is strictly necessary that the container has a perfect sealing capability.
The rear cover for the container which is forcibly torn in the above-mentioned manner has had a circular cutting line of a diameter slightly greater than the outer diameter of the piston 4, and the rear cover is broken at the cutting line to allow the central portion 7 defined thereby to be separated from the circumferential portion of the rear cover. Such a cutting line is formed by pressing the rear cover with a commonly-used metal mold.
Owing to the mechanical construction of the gun 1, a considerably high pressure can be applied to the piston 4. However, the construction of the gun 1 is very simple and the lever 2 is manually operated so that the magnitude of a pressure which can be applied to the piston 4 is limited to a certain level. Accordingly, it is desirable that the cutting line at which the central portion 7 is separated from the circumferential portion of the rear cover be formed such that the rear cover can be torn with as small an amount of force as possible. However, when the cutting line is excessively weak, the rear cover may be opened accidentally or the cutting line is broken partially by accident so that the interior of the container comes to be communicated with the atmospheric air. This often causes the material in the container to be hardened. Consequently, providing an excessively weak cutting line rather causes the extrusion cover to be exposed to an increased danger. Namely, it can be said that the reduction of extrusion force is not compatible with the increasing of the perfectness of sealing capability of the container.
An extrusion cover having a rear cover made of aluminum foil and a screw portion made of tinplate has also been proposed. However, this extrusion cover is also possibly opened by accident when the container is collided with an object or dropped.